Attention-Deficit/Hyperactivity Disorder (ADHD) continues to be one of the most prevalent childhood psychiatric disorders. Perceptual aspects of attention function in ADHD have been extensively studied by means of speed-based measures. However, taking into account the potentially impaired fine motor coordination and reduced ability to regulate the speed-accuracy trade-off often described in the ADHD-literature, reaction time based testing should be accompanied by accuracy-only tests which clearly distinguish perceptual from response-related components of attention. In the present study, we used an unspeeded verbal-report paradigm based on the Theory of Visual Attention (TVA, Bundesen, 1990) to measure parameters of visual attention in 24 children with ADHD, 20 clinical controls with autism spectrum disorder, and 55 healthy controls. TVA measures included visual processing speed, capacity of visual short-term memory, and the threshold of visual perception. In addition, we used a continuous performance test, the Dual Attention to Response Task (Dockree et al., 2006) to measure error rates, attentional lapses and reaction time. The two tasks were functionally specific and have previously proven sensitive to discrete group differences (Caspersen & Habekost, 2013). Children with ADHD showed significantly reduced visual processing speed compared to controls, whereas children with autism had a smaller short-term memory capacity. Moreover, compared to controls, ADHD was associated with significantly worse performance on several measures of sustained attention, including more errors of commission/omission, and more variation in reaction time. The only difference between clinical groups was a significantly higher frequency of attentional lapses among children with ADHD. Overall, the study demonstrates the strength of the TVA framework in specifying and quantifying the cognitive profile of ADHD. Results suggest a potential for TVA-based testing to contribute to differential diagnostics of co-morbid disorders, which is crucial for the on-going work of mapping clinically important endophenotypes.